1. Field of the Invention
The invention relates generally to a tubular connection for tubular members used in the oilfield. Specifically, the invention relates to a relatively thin walled connection for tubular members under conditions of high tension and high pressure yet fabricated of high strength relatively thin wall material used as production tubing or casing in deep severe service oil or gas wells.
2. Description of the Prior Art
Downhole conditions in wells reaching depths over fifteen thousand feet include pressures from fifteen thousand (15,000) to twenty-five thousand (25,000) psi and temperatures approaching five hundred (500) degrees fahrenheit. Downhole conditions are often characterized as severe when sweet gas C.sub.O 2 or sour gas H.sub.2 S is encountered. In order to withstand deep well and severe service conditions, production tubing and casing connectors are often fabricated from relatively thin walled high alloy steel that is not upset or cold formed. Such steel has very high strength and thus is not required to be as thick as low strength steels. Even where severe service conditions are not expected, there are instances where near maximum of tensile strength of the connection is required of non-upset or non-cold formed tubular members.
Thus, there has developed a need in the tubular connection industry for a connection design applicable for non-upset or non-cold formed tubular members adapted for service under deep and/or severe conditions or other conditions.
Prior art connection designs provided on ends of tubular members not having upsets or not processed by cold forming have been provided on a single thread geometry having a positive load flank thread. Such prior art design leads to radial crest to root thread interference causing significant assembly stresses and jumpout failures.
Straight or cylindrical two step thread designs are known in the prior art, but maximum tensile strength ratings of such designs may not be achievable especially where service of the tubing members is expected in extremely deep wells.
It is therefore an object of this invention to provide a tubular connection and pin and box members for that connection especially adapted for high-strength steel tubular members which are not upset or cold formed, and where the connection overcomes the disadvantages of the prior art connections.
It is a further object of the invention to provide a connection design which achieves a higher tensile rating than could be obtained with non-tapered two-stepped threads, that is where the stepped threads are both cylindrical threads.
It is a further object of this invention to provide a tubular connection which reduces the susceptibility of the connection to stress corrosion cracking in a severe service environment.
It is still another object of the invention to provide a connection which inhibits excessive compression forces from radially collapsing the end of the pin member.
It is another object of the invention to provide a connection which creates axial compression of the internal surface of the pin thereby reducing the susceptibility of the connection to stress corrosion cracking in a severe service environment.
It is another object of the invention to provide sealing surfaces which increase the seal radial load distribution over that provided by prior art connections thereby reducing the tendency for galling by reducing peak seal load.
It is another object of the invention to provide a connection which combats the tendency for jump-out by locking the pin and box together thereby reducing collapse of the pin and expansion of the box.